1. Field of the Invention
The invention relates to coolants containing metal corrosion inhibitors useful in water-cooled internal combustion engines.
2. Description of the Prior Art
Prior art corrosion inhibitors for ethylene glycol based antifreeze compositions are primarily of three types, the alkali phosphate, sodium benzoate-sodium nitrite, and alkanolaminophosphate. Borax is frequently added for adjusting the reserve alkalinity and other conventional metal corrosion inhibitors are added such as the silicates, nitrates, benzotriazole and benzothiazole. Metal corrosion inhibitors based upon alkali metal phosphates are of no importance in Europe since they provide insufficient protection against corrosion for light metals in highly stressed engines. Inhibitors based upon sodium benzoate, which usually contain silicate and benzotriazole in addition to borax, are primarily used in Europe. Generally, these commerical products meet the technological requirement of the current market.
Metal corrosion inhibitors based on alkanolaminophosphates are used in some cases in Germany, but are preferred in England and in the Scandanavian Countries. Their drawback lies in the fact that they are incompatible with hard water, resulting in the precipitation of calcium phosphate. Corrosion protection of iron and its alloys in the presence of alkanolaminephosphates is insufficient when the engine is exposed to high thermal stresses as the result of precipitation of iron phosphate which causes a loss of heat transfer efficiency. Toxic nitroso compounds may result upon mixing nitrite and alkanolamine phosphates. One object of the invention is to provide coolants based on ethylene glycol that can be mixed with other coolants based on ethylene glycol. The inihibitor component in the coolant compositions of the invention contain neither nitrite, amines nor phosphate and yet the coolants of the invention display good resistance to metal corrosion.
Although prior art coolants without added nitrite inhibitors do not yet meet metal corrosion protection requirements, particularly when used with aluminum engine parts, it was surprisingly found that the replacement of sodium nitrite by derivates of benzoic acid having strongly electron-attracting groups results in coolants which meet metal corrosion requirements. The use of nitrophenols as inhibitor components is described in German application No. 14 92 522 and U.S. Pat. No. 2,197,774. However, This previous literature neither suggests the use of ntrobenzoic acids nor is an even approximately comparable effect obtained with nitrophenols.